This invention relates to control of a sheet making process, and more particularly to a method for coordinating operation of machine direction and cross direction actuators in a sheet-making machine.
The control of sheet properties in a sheet-making machine is concerned with keeping the sheet properties as close to target values as possible. There are two sets of different actuators used for the control of the sheet properties. First, there are machine direction (MD) actuators that only affect the cross direction (CD) average of the sheet property. Each MD actuator can have different dynamic responses in the sheet properties. Second, there are CD actuators that are arrayed across the sheet in the CD. Each array of CD actuators can affect both the average and the CD shape of the sheet properties. CD actuators can have different dynamic responses and different spatial responses in the sheet properties. The problem of overall control of the sheet properties is highly multivariate: one CD actuator in a CD array affects adjacent CD zones in several sheet properties, and the average effect of a CD actuator array intended to control a particular sheet property can affect the average in several sheet properties which are also affected by several MD actuators. The problem is also one of very large scale. A typical control process can have several thousands of outputs (sheet property measurements) and several hundreds of inputs (actuator set points). The process is also difficult or impossible to control in certain spatial and intra actuator set directions.
Today in most conventional sheetmaking equipment, the control of sheet properties is separated into two control problems. First, the CD average is controlled only utilizing the MD actuators, not taking advantage of the CD actuators effect on the CD average of the sheet properties. Second, the CD actuators arrayed across the sheet only utilized to control the CD variation in around the average of the sheet properties. There are MD control schemes available today that utilizes model predictive control with explicit hard constraints handling for coordinating the MD actuators.
Optimal coordinated control of CD actuator arrays controlling one and multiple sheet properties using Model Predictive Control has been discussed in such articles as Backstrom J, Henderson B and Stewart C, xe2x80x9cIdentification and multivariable control of supercalendersxe2x80x9d Control Systems 2002, June 2002, Stockholm Sweden and Backstrom J. U, Gheorghe C, Stewart G. E, Vyse R. N xe2x80x9cConstrained model predictive control for cross directional multi-array processesxe2x80x9d. Pulp and Paper Canada. T128 102:5 (2001).
The need for coordinating MD actuators and CD actuators was identified in commonly owned U.S. Pat. No. 6,094,604 issued Jul. 25, 2000. A proposed solution to the problem was also disclosed in the ""604 patent involving a system of distributed localized intelligent controllers at the actuators that communicated with each other.
To address the issues outlined above, the present invention provides a flexible large scale Multivariable Model Predictive Controller for coordinated MD and CD control that takes multiple arrays of sheet property measurements as inputs and generates multiple arrays of outputs (actuator set points). The arrays can be of any dimension. An MD array is considered as a 1xc3x971 array. There can be any number of input and output arrays. The invention computes new optimally coordinated set points at evenly spaced control intervals. For each sheet property one can control the CD component only, the MD component only or both the MD and CD component. The inventions predicts the dynamic and spatial 2-dimensional response over a prediction horizon Hp to future Hc actuator set points where Hc is the control horizon. The invention then computes the future optimal set points that bring the future predicted sheet properties as close to target as possible. The controller also takes the physical limitations on the actuators into account explicitly. The controller handles the two types of directional problems by avoiding issuing actuator set points in the difficult spatial and intra actuator set process directions. This ensures closed loop 2-dimensional robust stability.
Accordingly, the present invention provides a process for coordinated control of machine direction MD and cross direction CD actuators in a sheetmaking machine for manufacturing a sheet of material comprising the steps of:
measuring a plurality of sheet properties at regular intervals to collect sheet measurement data;
manipulating the sheet measurement data to establish a plurality of sheet property measurement arrays;
processing the sheet property measurement arrays to establish a one dimensional common resolution measurement array
generating an array of the estimated current internal state of the sheet manufacturing process;
establishing a future sheet property target array;
generating an array of future predictions of sheet properties using the array of the estimated current internal state of the sheet manufacturing process and a sheet machine process model; and
inputting the array of future predictions of sheet properties, the future sheet property target array, and an array of previous actuator set points into an object function solvable to yield an array of optimal changes in the current actuator set points for coordinated MD and CD control of the sheet making process.
The present invention also provides a process for coordinated control of machine direction MD and cross direction CD actuators in a sheetmaking machine for manufacturing a sheet of material comprising the steps of:
measuring a plurality of sheet properties at regular intervals to collect sheet measurement data;
manipulating the sheet measurement data to establish a plurality of sheet property measurement arrays;
mapping the sheet property measurement arrays to a common resolution;
concatenating the common resolution sheet property measurement arrays into one larger one-dimensional common resolution measurement array;
generating an array of the estimated current internal state of the sheet manufacturing process by inputting the common resolution measurement array and an array of past changes in actuator set point to a sheet machine process model state observer;
concatenating a plurality of future sheet property target arrays into one target array;
generating an array of future predictions of sheet properties using the array of the estimated current internal state of the sheet manufacturing process and the sheet machine process model;
inputting the array of future predictions of sheet properties, the target array, object function weights, an array of the last actuator set points, and hard constraints into an object function; and
solving the object function to yield an array of optimal changes in the current actuator set points for coordinated MD and CD control of the sheet making process.
The present invention acts to optimally manipulate and coordinate the CD actuator arrays and the MD actuators in order to minimize the MD and CD variation in the sheet properties.
The invention optimally coordinates the interaction between MD actuator and CD actuator arrays. The invention further has a general weighting function in the objective function for expressing the cost of moving in small spatial gain directions. The invention further has an explicit weighting function for expressing the cost of moving in small intra actuator set directions. The invention further includes hard constraint specifying an allowable range for CD actuator array set point averages. The invention can be set up to control CD only, MD only or both the CD and MD components of a sheet property.
Preferably, the process of the invention uses one centralized controller rather than multiple distributed controllers.
The invention takes hard actuator constraints explicitly into account.